1. Field of the Invention
The present invention relates to a warm-up circuit for CCFLs and, more particularly, to a circuit disposed in a scanner to warm up and control the brightness of a CCFL according to a reference voltage.
2. Description of Related Art
Scanners are popular products in a fast developing industry. They can convert images like common photographs, texts and pictures into digital format that can be displayed, edited, stored and printed by a computer. Scanners still use cold-cathode fluorescent lamps (CCFLs) as their primary light sources. The CCFL generally needs a period of time to generate light of stable brightness after it is turned on. This period of time is referred to a warm-up time. That is, before a scanner starts a scanning operation, a warm-up time of about several minutes is required to let the CCFL have a uniform brightness after the power is turned on. The scanning operation can then be carried out.
As shown in FIG. 1, the y-axis represents the brightness of the CCFL in units of cd/m2, while the x-axis represents time t. Under a constant lamp current I of the CCFL, the brightness of the lamp will rise with the time t. When a warm-up time ts is reached, the brightness of the lamp will reach a stable state. For the above adjustment of the brightness of the lamp, the required constant lamp current is provided by an analog voltage controller. That is, an analog voltage is used to adjust the brightness.
After a user turns on the power of a scanner from the off state, it is necessary to wait for a warm-up time of several minutes before scanning. Moreover, when the scanner is used for continuous scanning, the constant lamp current continually flowing to the lamp will produce a lot of heat, which would cause structural deformation and would have negative effect on scanning quality. In other words, the warm-up time will have negative effect on the operation efficiency of a scanner by slowing the scanning speed, and heat produced by the lamp will have negative effect on the scanning quality. Therefore, shortening the warm-up time and controlling the constant lamp current will enhance the operation efficiency of the scanner and reduce heat produced by the lamp to avoid structural deformation and also improve the scanning quality.